This invention is in the field of stents for implantation into a vessel of a human body.
Stents are well known medical devices that have been used for maintaining the patency of a large variety of vessels of the human body. The most frequent use is for implantation into the coronary vasculature. Although stents have been used for this purpose for more than ten years, many stent designs still lack the required flexibility and radial rigidity to provide an optimum clinical result. Another deficiency of open cell stents is that some stent struts members can flare outward (fish scaling) as the stent is advanced through a tight curve.
An open cell stent is defined as a stent that has circumferential sets of strut members with some curved sections (crowns) that are not connected by a longitudinal connecting link to an adjacent circumferential set of strut members. In comparison, a closed cell stent has every curved section of every circumferential set of strut members, except at the distal and proximal ends of the stent, attached to a longitudinal connecting link. A strut member whose curved section is not attached to a longitudinal connecting link is defined as an unconnected strut member.
There are several xe2x80x9copen cellxe2x80x9d stents that are currently being marketed for the treatment of coronary stenoses. Examples of these are the Tetra stent from Guidant Corporation and the S670 stent from Medtronics, Inc. Each of these stents has a limited number of straight longitudinal connecting links to join adjacent curved sections of adjacent circumferential sets of strut members. These straight longitudinal connecting links can cause outward flaring of the end circumferential sets of strut members as the stent bends around a curve. The interior unconnected strut members also can flare outward when the pre-deployed stent mounted on a balloon is advanced through a curved vessel such as a coronary artery. Any strut that flares outward can engage the vessel wall during stent delivery in a curved vessel thereby preventing the stent from reaching the site that is to be stented.
Most current open cell stents use a multiplicity of circumferential sets of strut members connected by straight longitudinal connecting links. The circumferential sets of strut members are typically formed from a series of diagonal sections connected to curved sections forming a closed-ring, zig-zag structure. This structure opens up as the stent expands to form the element in the stent that provides structural support for the arterial wall. A single strut member is defined as a diagonal section connected to a curved section within one of the circumferential sets of strut members. In current open cell stent designs such as the Tristar stent (Guidant Corp.) these sets of strut members are formed from a single piece of metal having a uniform wall thickness and uniform strut width. Although a stent with uniform width of the strut members will function, if the width is increased to add strength or radiopacity, the sets of strut members will experience increased strain upon expansion. High strain can cause cracking of the metal and potential fatigue failure of the stent under the cyclic stress of a beating heart.
Existing highly radiopaque stents such as the Crossflex coil stent (made from 0.005 inch diameter tantalum wire) by Cordis Corp. and the gold plated NIROYAL stent made by the Boston Scientific Co. can obscure the inside of the vessel because of the high radiopacity over the entire length of the stent. The Be stent of Medtronics, Inc., has small gold markers at the ends of the stent but those markers only mark an end point without allowing visualization of the entire end set of strut members. Fischell et al in U.S. Pat. No. 6,086,604 teaches a closed cell stent with the end sets of strut members being gold plated. Such a stent would have ideal radiopacity but could (like the Be stent) exhibit corrosion due to dissimilar metals placed in an electrolytic solution such as the blood. There has also been significant evidence that gold is a poor surface material for stents because it can increase the risk of subacute thrombosis and restenosis.
Fischell et al in U.S. Pat. No. 5,697,971 show in FIG. 7 a stainless steel stent with increased width diagonal sections in all the circumferential sets of strut members. The U.S. Pat. No. 5,697,971 does not teach the use of a variable width for diagonal sections as a means to tailor the stent radiopacity. The U.S. Pat. No. 5,697,971 also does not teach specific metals and alloys having greater radiopacity than stainless steel, nor does it teach the use of the alternating curved sections and diagonal sections to form the circumferential sets of strut members. Furthermore, the U.S. Pat. No. 5,697,971 does not teach the use of variable width of curved sections that can provide additional radial rigidity and/or variable width diagonal sections to create relatively greater radiopacity for the end set of strut members.
The present invention is an open cell stent that is designed to optimize many of the operating parameters that are expected for stents in the first decade of the 21st century. Specifically, an optimum stent design would have the following characteristics:
I. IN THE PRE-DEPLOYED STATE
1. excellent flexibility
2. low profile (i.e.; small outside diameter of the stent)
3. good radiopacity
4. smooth outer surface
5. no flaring of struts when advancing through curved arteries
6. a high degree of stent retention onto the delivery catheter
II. AFTER DEPLOYMENT
1. flexible so as to conform to a curved artery
2. radially rigid (i.e.; low recoil)
3. good radiopacity
4. good coverage of the vessel wall (i.e.; no plaque prolapse)
5. side branch access without strut breakage
6. minimal foreshortening compared to the length of the stent in its pre-deployed state
Although many desirable attributes are required of the catheter that is used to deliver the stent, the scope of the present invention is limited to the design of the stent itself However, it should be understood that the reduced foreshortening of this stent is a result of having undulating longitudinal connecting links that readily extend in their longitudinal length when the balloon onto which the stent is crimped is inflated.
To accomplish the goals listed in I. and II. above, the stent would optimally have at least two open cells around the circumference of the stent. A unique feature of the present invention is that each of the strut members whose curved sections are unconnected has a shorter longitudinal length as compared to the longitudinal length of the strut members that are connected by a longitudinal connecting link. This shorter length (optimally on the order of at least 0.1 mm shorter) reduces outward flaring of the unconnected strut members when the stent is advanced through highly curved vessels such as some coronary arteries. Flaring (which is sometimes called xe2x80x9cfish-scalingxe2x80x9d) can cause the stent to engage the vessel wall as the stent is advanced through curved arteries thus precluding delivery of the stent to its intended location.
Another novel feature of this stent is that the longitudinal connecting links can have an undulating shape so that they can easily expand or contract in their longitudinal length when the stent is advanced through a curved vessel. The extraordinary capability of this stent to bend easily is a combination of the fact that those curved sections of adjacent circumferential sets of strut members that are connected are connected with flexible longitudinal connecting links, and many (typically one-half) of the curved sections are unconnected. Of course, the weakest possible connection that provides the highest degree of longitudinal flexibility is being unconnected. Therefore, the combination of no connections plus the few required connections between the circumferential sets of strut members being by means of highly flexible undulating longitudinal connecting links imparts to this stent an extraordinarily high degree of longitudinal flexibility.
It should also be understood that all the strut members at each end of the stent should also have a shortened longitudinal length because the outside curved section of the end circumferential sets of strut members cannot be connected to any adjacent circumferential set of strut members. By shortening all the end strut members, end flaring of the stent as it is advanced through curved vessels can be reduced. Furthermore, the fact that the interior curved sections of each strut member at the ends of the stent either have no connection or a flexible, undulating longitudinal connecting link connection to an inner strut members is desirable in preventing flaring out of the strut members at the ends of the stent. This is not the case for strut members that have a straight connection to an end circumferential strut such as shown in FIG. 5 of U.S. Pat. No. 5,759,192.
Good radiopacity for the stent is achieved by having a stainless steel stent that has a wall thickness that is the least 0.0045 inches. Another means would be to use a metal with a higher density such as tantalum with a thickness greater than 0.002 inches. A third means for obtaining improved radiopacity would be to sandwich a high density metal between two layers of stainless steel with each of the co-axial tubes having a wall thickness between 0.001 and 0.002 inches.
Another feature of the present invention is that the undulating longitudinal connecting links readily extend in the longitudinal direction when the balloon is inflated. Since the circumferential sets of strut members upon deployment tend to decrease in their longitudinal length, the longitudinal lengthening of the undulating longitudinal connecting links has the effect of minimizing the foreshortening of the deployed stent.
To optimize radiopacity, the ideal radiopaque, open cell stent would have end sets of strut members that are highly radiopaque so that they can be readily seen even using low power fluoroscopy with an interior section of the stent that is visible but not too bright so as to obscure the lumen when high power cine film angiograms are taken. The stent should also have only one material on its outside surface to avoid potential corrosion and that material should not promote subacute thrombosis or restenosis.
The present invention is an open cell stent stent that is designed to have optimal strength and radiopacity with good biocompatibility. Unfortunately, the choices of appropriate biocompatible metals available as thin wall tubing for stent construction are somewhat limited. To achieve optimal radiopacity, the stent design of the present invention is adjusted to the specific radiopacity and strength characteristics of the metal from which the stent is fabricated. What is more, coatings such as parylene may be needed to avoid corrosion from stents with less biocompatible materials and/or dissimilar metals on the stent""s exterior surface. Of extreme importance to the present invention is the achievement of optimal radiopacity in a stent that is only 0.004 inches wall thickness or less. Such a stent would have a pre-deployment outer diameter (profile) that would be at least 0.003 inches less than the open cell Tristar stent (Guidant Corp.) that has a wall thickness of 0.0055 inches. Ideally, the stent described herein would have a wall thickness between 0.0025 inches and 0.004 inches.
Described below are the novel design elements for open cell stents formed from stainless steel or a highly radiopaque metal such as tantalum, or plated with highly radiopaque materials like gold.
1. Tapered Strut Width for Stents Formed From Highly Radiopaque Metals
Although reducing the width of the diagonal section alone will reduce radiopacity without significantly affecting radial strength, by having a taper on the curved sections of the circumferential sets of strut members, a greatly reduced level of strain upon stent expansion can be achieved without sacrificing radial strength. This is extremely important as it allows a stent to be made much stronger than a stent with uniform width of the strut members while staying within the same strain limit for the material.
Tantalum is a metal that is used for stents and it is highly radiopaque. The optimal radiopacity for a stent design using tantalum could have uniform width for the circumferential sets of strut members and a wall thickness of 0.0025 inches. To provide more radial strength and to reduce the probability of the stent ends flaring out during deployment, a wall thickness of 0.003 inches to 0.035 inches would be highly desirable. With uniform width sets of strut members, a 0.035 inches wall thickness tantalum stent would be too bright under cine angiography. To reduce the radiopacity of the design without significantly impacting the radial strength of the deployed stent, the present invention envisions curved sections and diagonal sections either or both of which could have a variable or tapered width. The curved sections should be tapered (wider at the center compared to the ends) to reduce strain as previously described. The diagonal sections can be thinner in the center than at the ends to reduce radiopacity for the interior sets of strut members while maintaining a higher degree of radiopacity for the end set of strut members. It is envisioned that the novel open cell stent described herein might have wider diagonal sections for the end sets of strut members as compared to the interior sets of strut members. This feature would enhance the radiopacity of the end sets of strut members while retaining a moderate level of radiopacity for the interior sets of strut members. It is also envisioned to have both reduced width diagonals and/or reduced wall thickness for the interior sets of strut members. It is fluoroscopic visualization of the end sets of strut members that is most important for visualizing stents placed inside a coronary artery. Therefore, the end sets of strut members should have either or both a greater width of diagonal sections or a greater strut thickness as compared to those parameters of the interior sets of strut members.
2. End Sets of Strut Members with Thinner Curved Sections
Stent deliverability into curved coronary arteries is improved when the diagonal sections of the end sets of strut members have a decreased length as compared to the length of the diagonal sections of the interior sets of strut members. A shorter length of the diagonal sections will also reduce outward flaring upon expansion of the stent. Decreasing end flaring of the deployed stent is of particular importance for stents having very thin walls. Burpee and Fischell in U.S. Pat. No. 5,913,895 describe a stent with shorter diagonal sections in the end sets of strut members. The design of Burpee and Fischell is limited by the strain limit allowed for the end sets of strut members. As a result, if the end sets of strut members are made as strong as possible while being limited by the maximum allowable strain for that metal, the interior sets of strut members will not have optimized radial strength. The present invention envisions optimizing the radial strength for all circumferential sets of strut members for an open cell stent; i.e., the metal in all sets of strut members reaches the same, maximum allowable strain at the limiting diameter for the stent""s expansion. To achieve this desired attribute, the stent described herein has curved sections of the end sets of strut members that are less wide than the curved sections of the interior sets of strut members.
3. Good Sidebranch Access While Maintaining Small Cell Size
The stents described herein are open cell stents having every other curved section of every interior set of strut members connected to an adjacent set of strut members by an undulating longitudinally extending link. Open cell stents connected with such an undulating connecting link should have the interior length of the perimeter of each of the stent""s cells to be at least 9 mm in length. This design parameter allows each cell of the stent to be expanded to a circular diameter of approximately 3 mm (i.e., 9/xcfx80≅3). This feature allows the xe2x80x9cunjailingxe2x80x9d of side branches of the artery into which the stent is placed. A cell for good side branch access should have an interior perimeter length between 9 mm and 15 mm. (i.e., an expandable circular diameter between 2.86 and 4.8 mm).
4. Flexible Undulating Longitudinal Links With Good Support Between Adjacent Sets of Strut Members
To provide a strong bridge connection between adjacent circumferential sets of strut members, the flexible undulating longitudinal connecting links should have nearly equal extent in the circumferential direction on each side of a line drawn between the attachment points of the flexible undulating longitudinal connecting link to the curved sections of adjacent sets of strut members. xe2x80x9cSxe2x80x9d, xe2x80x9cNxe2x80x9d and inverted xe2x80x9cNxe2x80x9d shapes for the connecting links inherently have equal circumferential displacement on each side of the line connecting their attachment points. It is envisioned that xe2x80x9cMxe2x80x9d or xe2x80x9cWxe2x80x9d shaped or other types of connecting links can be used if there is an equal circumferential extent of these undulating links on each side of a line that joins the attachment points of these links to the sets of strut members.
5. Variable Thickness Radiopaque Coatings
The NIROYAL stent has a uniform thickness of gold plating, which makes the center too radiopaque as compared to the radiopacity of the end sets of strut members. The Fischell et al invention described in U.S. Pat. No. 6,086,604 teaches stents having gold placed only on the end sets of strut members. This creates a potential for corrosion from dissimilar metals, namely, gold and stainless steel. The present invention envisions a gold coating that is sufficiently thick on the end sets of strut members to provide optimal radiopacity with a thin coating of gold on the rest of the stent. This design prevents obscuring of the arterial lumen while providing an exterior surface for the stent that is a single metal thus avoiding electrolytic corrosion.
6. Polymer Coatings for Stents Coated With Gold or Having Dissimilar Metal Surfaces
For stents with non-biocompatible or dissimilar metals, the present invention envisions the use of a polymer such as parylene to coat the entire outer surface of the stent. This would improve biocompatibility and also allow attachment of organic compounds such as heparin or phosphorylcholine for reduced thrombogenicity or drugs such as Taxol or Rapamycin for reduced cell proliferation and a decreased rate of restenosis. It is also known that highly radiopaque materials like tungsten can be mixed into polymers. A stent coating including a plastic with mixed in radiopaque metal could be used to enhance both radiopacity and biocompatibility. Such a polymer coating could also be advantageous with a gold-coated stent.
7. Attachment of a Drug Coating
It is also envisioned that any one of the following drugs could be attached to the stent to decrease restenosis and/or subacute thrombosis: Rapamycin, Alkeran, Cytoxan, Leukeran, Cis-platinum, BiCNU, Adriamycin, Doxorubicin, Cerubidine, Idamycin, Mithracin, Mutamycin, Fluorouracil, Methotrexate, Thoguanine, Toxotere, Taxol, Etoposide, Vincristine, Irinotecan, Hycamptin, Matulane, Vumon, Hexalin, Hydroxyurea, Gemzar, Oncovin and Etophophos.
8. Providing a Variable Wall Thickness
The present invention also envisions next generation manufacturing techniques using photoetching whereby a stent pattern is etched into a thin-walled metal tube. These techniques already can produce variations in wall thickness as well as strut width for any stent pattern. The present invention envisions use of these techniques to create stents with optimal radiopacity. In particular for a stent formed from a single metal or alloy, thicker metal (i.e., greater wall thickness) at each end of the stent could increase radiopacity there as compared to the interior section of the stent.
It is also envisioned that any of the above optimal radiopacity stent designs may be used with plastic coatings such as parylene, antithrombogenic coatings such as heparin or phosphorylcholine or anti-proliferative coatings such as Taxol or Rapamycin, or any other drug as listed above.
Thus an object of the present invention is to have increased longitudinal flexibility for the stent by having some curved sections of each circumferential set of strut members being unconnected to the curved sections of the adjacent circumferential set of strut members with the other curved sections being connected by highly flexible, undulating longitudinal connecting links.
Another object of the present invention is to prevent flaring of the unconnected strut members by having a shorter longitudinal length for the unconnected strut members.
Still another object of the invention is to have generally shorter longitudinal lengths for all the strut members in the circumferential sets of strut members at the stent ends to reduce the tendency for end flaring of the stent.
Still another object of the invention is to decrease the propensity of the stent to have end flaring by having each curved section of the end set of strut members either being unconnected to the curved section of the adjacent (interior) set of strut members or being connected by a highly flexible undulating longitudinal connecting link.
Still another object of the invention is to connect the flexible longitudinal connecting links to the curved sections of the sets of strut members at a point that is close to the line where the diagonal sections join the curved sections, thus further reducing the propensity for end flaring.
Still another object of the invention is to have flexible longitudinal connecting links that are adapted to readily increase their longitudinal length when the balloon is inflated; thus minimizing the extent of stent foreshortening upon stent deployment.
Still another object of the present invention to have a stent that has sets of circumferential strut members with tapered curved sections, the center of the curved sections being wider than the ends of the curved sections so as to reduce plastic strain as the stent is expanded as compared to a curved section with uniform width.
Still another object of the present invention is to have a stent with tapered diagonal sections in the sets of strut members where the center of the diagonal section is narrower than the ends of the diagonal section to reduce the radiopacity of interior sets of strut members of the stent as compared to a stent with diagonal sections having a uniform width.
Still another object of the invention is to have a stent with decreased wall thickness of the interior set of strut members as compared to the end set of strut members so as to have a comparatively higher radiopacity for the end sets of strut members.
Still another object of the present invention is to have a stent having end sets of strut members with tapered diagonal sections that are wider at their center compared to their ends to increase the radiopacity of the end sets of strut members.
Still another object of the present invention is to have end sets of strut members having both shorter diagonal sections and thinner width curved sections as compared to those sections in the interior sets of strut members.
Still another object of the present invention is to have a tantalum stent with wall thickness less than 0.035 inches having tapered sets of strut members whereby the diagonal sections of the interior sets of strut members are less wide than the width at the center of the curved sections.
Still another object of the present invention is to have an open cell stent design with maximum post-deployment interior cell perimeter length that is equal to or greater than 9 mm.
Still another object of the present invention is to have a stent with a radiopaque metal coating where the radiopaque metal coating has greater wall thickness on the end sets of strut members as compared to thickness of the radiopaque metal coating on the interior sets of strut members.
Still another object of the present invention is to have an open cell stent design with xe2x80x9cSxe2x80x9d or xe2x80x9cNxe2x80x9d or inverted xe2x80x9cNxe2x80x9d or other shaped flexible undulating longitudinal connecting links wherein the circumferential extent of each flexible undulating longitudinal connecting link is approximately equal on each side of a line drawn between the proximal and distal attachment points of the flexible undulating longitudinal connecting link.
Still another object of the present invention is to have the stent with optimized radiopacity formed with an outer surface that is plastic coated to improve biocompatibility.
Still another object of the present invention is to have the stent with optimized radiopacity that is coated with a plastic material and an additional material to prevent thrombus formation and/or restenosis; the additional material being selected from the group that includes Rapamycin, Alkeran, Cytoxan, Leukeran, Cis-platinum, BiCNU, Adriamycin, Doxorubicin, Cerubidine, Idamycin, Mithracin, Mutamycin, Fluorouracil, Methotrexate, Thoguanine, Toxotere, Taxol, Etoposide, Vincristine, Irinotecan, Hycamptin, Matulane, Vumon, Hexalin, Hydroxyurea, Gemzar, Oncovin and Etophophos.
Still another object of the present invention is to have a stent coated with a plastic material that includes a radiopaque filler material.